The present invention relates to electron discharge devices and more particularly to phototubes such as photomultiplier tubes.
A phototube is an electron discharge device which is particularly useful for detecting an input signal in the form of radiation which impinges on an input surface of the device. A photocathode converts the impinging radiation into a stream of electrons which is ultimately collected by an anode to produce an electrical output signal related to the real time magnitude of the collected electron stream.
In photomultipliers, an electron multiplier is interposed between the photocathode and anode to provide, in ordered sequence, one or more electrode stages of electron multiplication. An electric field between the photocathode and the succeeding electrodes acts as an electron lens whereby the various electrons of the electron stream are accelerated within an evacuated cavity to impinge upon each of the succeeding electrodes in ordered sequence.
Phototubes have generally been limited to their ability to uniformly convert and amplify information in the form of an incident radiation signal event into a useful signal output independent of its point of incidence along the input surface. For example, undesirable variations in signal output level and pulse height resolution are known to occur depending upon the point of incidence of the signal event along the input surface and the photocathode associated therewith. One reason for this deficiency has been an inability of one or more electrodes to completely and uniformly collect all the electrons emitted from the entire effective photocathode surface. Prior art electron lens systems are generally unable to provide the desired optimum focussing of the entire electron stream. As a consequence, a significant percentage of the electrons which are emitted from the peripheral portions of the cathode do not result in a useful anode signal output.